Call by reference: Use of undeclared identifier

Question

I try to return multiple values by reference.

enum Color {ORANGE, YELLOW, GREEN, BLUE, VIOLET, RED};
int main(int argc, const char * argv[])
{
    CvScalar hsv_min, hsv_max;
    choose_color(RED, hsv_min, hsv_max);

    return 0;
}

void choose_color(Color farbe, CvScalar &min, CvScalar &max) {
    switch (farbe) {
        case ORANGE:
            min = cvScalar(0,50,50);
            max = cvScalar(0,255,255);
            break;
        default:
            throw "choose color: invalid case!";
            break;
    }
 };

Why do I get "use of undeclared identifier" for the choose_color call?

score 2 · Accepted Answer · edited May 23 '17 at 11:51

2

Because your main function needs to know that your choose_color exists.

You need to do a forward declaration (place the prototype of the function before the main function):

enum Color {ORANGE, YELLOW, GREEN, BLUE, VIOLET, RED};

// Forward declaration
void choose_color(Color farbe, CvScalar &min, CvScalar &max); // function prototype

int main(int argc, const char * argv[])
{
    CvScalar hsv_min, hsv_max;
    choose_color(RED, hsv_min, hsv_max);

    return 0;
}

void choose_color(Color farbe, CvScalar &min, CvScalar &max) {
// ...
};

Here is great explanation about forward declaration.

The other solution is to define the main after your choose_color function.

edited May 23 '17 at 11:51

Community

1
1

answered Sep 10 '13 at 18:27

Pierre Fourgeaud

14,290
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I'd note that this is often referred to as "prototyping" the function. – BrainSteel Sep 10 '13 at 18:29
@BrainSteel I edited my post to talk about prototype. Thanks for the suggestion ! – Pierre Fourgeaud Sep 10 '13 at 18:30
@PierreFourgeaud You need to move the forward-declaration of `choose_color` *after* the definition of `Color`. – Daniel Frey Sep 10 '13 at 18:34
@DanielFrey Outch ... That's true... I fixed it. Thank you for pointing this out ! – Pierre Fourgeaud Sep 10 '13 at 18:35
In general what is preferred? Forward declaration or definition of main after all functions? – bro Sep 10 '13 at 19:20

score 0 · Answer 2 · answered Sep 10 '13 at 18:26

0

place main() after choose_color() or, better, declare the function above main():

void choose_color(Color farbe, CvScalar &min, CvScalar &max);
int main(int argc, const char * argv[])
{
...
}

answered Sep 10 '13 at 18:26

CapelliC

59,646
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Call by reference: Use of undeclared identifier

2 Answers2